I'm presuming that you have Satellite TV with Sky UK and that you have their Devialet sound box.
The only way you can connect the Devialet box using HDMI is via your Sky box. You have to connect your Devialet box to your Sky box using HDMI and then a HDMI cable to your TV. The option to connect the Devialet box to your TV directly using HDMI using HDMI arc is non existent. It has to go through a Sky box. Your Devialet box will turn on on its own when you turn on your Sky box.
If you don't have Sky or you want to push sound through to your Devialet box from another device, like blu ray player, Xbox etc, you will have to use optical cable from the Devialet box to your TV.
If you have used the optical sound setting and then turn on to go back to a device other than your Sky box, the Devialet box should stay on the optical output. For example, if you've been playing Xbox and you turn everything off, when you go back to play Xbox again and turn it on along with your TV, the Devialet box will already be at the optical sound output. If, however, you do have Sky TV and you turn the Sky box on, your Devialet box will revert to HDMI and get sound from your Sky box only.
If you don't have Sky TV at all, your only option is to use optical cable but you'll have to turn on the Devialet box independently each time you want to use it, but it will be at the optical output setting when it's turned on each time.
I hope this make sense.